The invention relates to a method of estimating the signal-to-noise ratio of a signal received by a radiocommunications receiver. It also relates to a receiver for implementing the method and to an application of the method to controlling the power of a transmitter.
A telecommunications system generally transmits a large number of different calls simultaneously.
By way of example, the telecommunications system considered herein is one in which terminals communicate with a control or connection station, in particular via retransmission means on a satellite. Calls between terminals are effected via the control station. The control station therefore communicates simultaneously with a set of terminals.
In the above telecommunications system, the number of calls that can be transmitted simultaneously depends on the retransmission power available on the satellite. To maximize the capacity of the system, in other words to maximize the number of calls that can be transmitted simultaneously, it is necessary to minimize the power allocated to each transmitter, because the retransmission power is necessarily limited. However, this constraint is difficult to reconcile with the requirement to optimize call quality, which requires sufficient transmission power. As a general rule, the calls are digital calls and transmission quality is assessed against a maximum permitted error rate. The permitted error rate is guaranteed if the received signal-to-noise ratio is above a pre-determined threshold.
Thus the power of a transmitter is generally determined from the signal-to-noise ratio measured at the associated receiver and the signal-to-noise ratio is generally measured continuously, in particular in a satellite transmission system, because propagation conditions can vary, in particular because of variations in meteorological conditions. For example, rain causes strong attenuation of the received signal compared to transmission in fine weather. Propagation conditions can also be degraded by scintillation due to multiple signal paths causing additive and subtractive combination of signals. Propagation conditions can also be degraded because of masking when an antenna is tracking a mobile source (here the satellite) and obstacles block the path of the transmitted signal.
The measured signal-to-noise ratio of a received signal is itself generally subject to estimation noise and the measurements are usually smoothed, for example by low-pass filtering, to reduce the estimation noise.
The accuracy of the measured signal-to-noise ratio determines the capacity of the telecommunications system. If the measurement is accurate, each transmitter is allocated just the necessary power, which therefore maximizes the communications resources, whereas an inaccurate measurement leads to the allocation of too much power to each transmitter, which is not favorable to maximizing communications capacity.